Vehicle evaporative emission control systems may be configured to store refueling vapors, running-loss vapors, and diurnal emissions in a fuel vapor canister, and then purge the stored vapors during a subsequent engine operation. The stored vapors may be routed to engine intake for combustion, further improving fuel economy for the vehicle. In a typical canister purge operation, a canister purge valve coupled between the engine intake and the fuel vapor canister is opened, allowing for intake manifold vacuum to be applied to the fuel vapor canister. Fresh air may be drawn through the fuel vapor canister via an open canister vent valve. This configuration facilitates desorption of stored fuel vapors from the adsorbent material in the canister, regenerating the adsorbent material for further fuel vapor adsorption.
However, air flow through the fuel vapor canister is not typically uniform, and thus the canister may never be purged out completely. Furthermore, while hydrocarbon light ends (e.g., propane, butane) are easily desorbed by drawing fresh air through the fuel vapor storage canister, heavy ends (e.g., heptane, octane) are more difficult to desorb. For example, heavy ends may require a motive force in the form of heat applied to the fuel vapor storage canister for desorption. As such, regions of the canister that are not properly cleaned may contribute to bleed emissions when a vehicle is parked for a duration in a climate that may contribute to heat-induced desorption of residual hydrocarbon vapor. Such issues are particularly relevant in hybrid electric vehicles (HEVs), or other vehicles with limited engine run time, as with limited engine run time, opportunities for canister purging are in turn also limited.
US 20130152905 A1 teaches a hydrocarbon sensor in an extended range electric vehicle configured to determine when a fuel vapor canister is saturated with fuel vapors, such that the engine is only activated when needed in order to purge the fuel vapor canister and thus prevent bleed emissions. However, the inventors herein have recognized potential issues with such an approach. For example, if the hydrocarbon sensor fails, then undesired evaporative emissions may be released to the environment. Furthermore, as the hydrocarbon sensor is configured to indicate when the canister is saturated with fuel vapors, such a hydrocarbon sensor may in some examples never see hydrocarbon vapors until breakthrough occurs, thus resulting in no potential opportunities to rationalize the functionality of the hydrocarbon sensor. If the hydrocarbon sensor is not functioning properly at the time of breakthrough, undesired evaporative emissions may result.
Furthermore, various strategies have been proposed for electrical heating of the fuel vapor storage canister to improve desorption of fuel vapors stored in a fuel vapor canister. However, the inventors have herein additionally recognized potential issues with such approaches. For example, as light ends are readily desorbed by the drawing of fresh air across the fuel vapor canister, heating of the canister to promote desorption of both hydrocarbon heavy ends and light ends may be undesirable due to the amount of electrical power thus required. A desirable alternative would involve utilizing a canister heater to specifically desorb hydrocarbon heavy ends, for example. In addition, under conditions where a fuel vapor canister heater may be utilized, it may be desirable to indicate whether the fuel vapor canister heater is functioning as desired.
Thus, there is a need to provide an ability to rationalize a hydrocarbon sensor in order to reliably assess whether bleed emissions are occurring from the fuel vapor canister. Furthermore, there is a need to be able to thoroughly clean the fuel vapor canister, without requiring an excessive amount of electrical power. Additionally, in a case wherein a fuel vapor canister heater is utilized, there is a need to reliably assess whether the canister heater is functioning as desired. The inventors herein have recognized these issues, and have developed systems and methods to at least partially address such issues. In one example, a method is provided, comprising capturing and storing fuel vapors in a fuel vapor storage canister positioned in a vehicle evaporative emission system, the fuel vapor canister removably coupled to a fuel tank that provides fuel to an engine that propels the vehicle; actively routing fuel vapors from the fuel vapor canister into a vent line coupling the fuel vapor canister to atmosphere; and diagnosing one or more evaporative emission system components responsive to the routing.
As one example, actively routing fuel vapors from the fuel vapor canister into the vent line includes activation of a canister heating element coupled to and/or within the fuel vapor canister to promote desorption of fuel vapors stored in the fuel vapor canister. During the actively routing the fuel vapors into the vent line, the vent line may be monitored via a hydrocarbon sensor for the presence of fuel vapors subsequent to activation of the canister heating element, wherein diagnosing one or more evaporative emission system components includes indicating that both the canister heating element and the hydrocarbon sensor are functioning as desired responsive to an indication of the presence of fuel vapors in the vent line.
As another example, the method includes purging fuel vapors to an intake manifold of the engine, wherein the purging is conducted responsive to either an indication of fuel vapors in the vent line, or responsive to a predetermined time duration elapsing subsequent to activation of the canister heating element without an indication of fuel vapors in the vent line. In some examples, purging fuel vapors to the intake manifold of the engine prior to activating the canister heating element promotes desorption of hydrocarbon light ends from the fuel vapor canister, wherein diagnosing the canister heating element and hydrocarbon sensor is not conducted during or prior to purging of hydrocarbon light ends. In such an example, activating the canister heating element subsequent to purging the fuel vapor canister of hydrocarbon light ends serves to promote desorption of hydrocarbon heavy ends stored within the fuel vapor canister that are not purged during purging of the fuel vapor canister in the absence of activation of the canister heating element. By selectively purging the fuel vapor canister to desorb hydrocarbon light ends without activation of the canister heating element, and subsequently activating the canister heating element to desorb hydrocarbon heavy ends, thorough canister cleaning may be accomplished, battery power may be conserved, and it may be determined whether the canister heating element and hydrocarbon sensor are functioning as desired.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.